1. Field of the Invention
This invention relates to a wheel speed measuring circuit, and more particularly to a wheel speed measuring circuit which is most suitable for a skid control system for a vehicle.
2. Description of the Prior Art
Recently, various kinds of vehicle skid control arrangements have been developed to effect a skid control operation by means of which a locked wheel condition can be avoided and good braking performance can be obtained on any road. In all of these skid control arrangements, a rotational condition or skid condition of the wheel, such as deceleration, slip and acceleration, is measured by a control unit which receives a signal detected by a wheel speed sensor which is associated with a wheel for detecting the rotational speed of the wheel. The brake fluid pressure to the brake for the wheel is controlled on the basis of the measurement of the control unit.
In the control unit, the deceleration, slip and acceleration of the wheel are measured or calculated from the wheel speed signal detected by the wheel speed sensor. To calculate the slip of the wheel, an approximate vehicle speed is simulated on the basis of the wheel speed signal. The calculated decelerated and acceleration are compared with predetermined reference deceleration and acceleration to generate deceleration and acceleration signals, and the calculated slip is compared with a predetermined reference slip to generate a slip signal. The brake fluid pressure to the brake for the wheel is controlled with the deceleration and acceleration signals, and slip signal. The basic value for the skid control operation is a wheel speed.
Recent skid control systems have a tendency to be digital. The sinusoidal output of the wheel speed sensor associated with the wheel is wave-shaped to obtain rectangular pulses. The frequency of the pulses is proportional to the wheel speed. Accordingly, the wheel speed is measured on the basis of the pulses. In order to measure the wheel speed, hitherto, the pulses are counted for a predetermined time, or clock pulses of high frequency are counted for time intervals between the successive pulses from the wheel speed sensor. The time interval between the successive pulses is in reverse proportion to the wheel speed. However, in the former measuring method, the predetermined time should be considerably long so as to rise the measurement accuracy of the wheel speed, and therefore, a long time is required for measuring the wheel speed. In the latter measuring method, the measurement accuracies between a high wheel speed and a low wheel speed are greatly different from each other.
Further in both of the above measuring methods, complicated circuits are required for differentiating the measured wheel speed for obtaining the deceleration and acceleration of the wheel.